This invention relates to a method and apparatus for making castings called ingots, especially cylindrical ones.
It is well-known that there are two ways of casting molten metal; top pouring and bottom pouring.
Top pouring is a method that has been in use for a long time. One of the shortcomings of top pouring is that liquid metal flows out of a ladle at high speed because of the high static pressure in the ladle. On being poured into a mold, the metal violently strikes the stool at the bottom of the mold, scattering in the form of droplets. Part of the droplets adheres to the inner walls of the mold which deteriorates the surface of the ingots. The other part, repelled by the inner walls, drops into the liquid metal pooled in the mold. The repelled droplets become oxidized in the atmosphere, so that the iron oxide at the surface of the droplets and the carbon in the molten metal react with each other to evolve gases which, in turn, form blowholes.
To keep the liquid metal hot, a heat-retaining agent or the like is usually added to the mold. Such additives are often swallowed up by the pouring stream, giving rise to slag inclusions and other similar defects.
For these reasons, splash-preventing stools, cylinders or other devices have been employed but none are 100 percent satisfactory.
Bottom pouring is one of the methods which have come into increasing use to overcome the disadvantages of top pouring.
In bottom pouring, the flow energy of the pouring stream is reduced by the resistance offered by the fountain and runner which connect the ladle and the mold. Entering through the outlet at the bottom of the mold, the molten metal rises very steadily and quietly in the mold, without causing splashing. The result is a good ingot surface.
To produce clean ingots with uniform structure, various kinds of special ingot-making methods have come into use.
Rotary ingot-making is one example. According to this method, liquid metal is poured into a mold which is turned around on a rotating plate, the rotation being continued until the metal solidifies to a desired thickness. Accordingly, non-metallic inclusions with low specific gravity move toward the center and float up to provide greater opportunity to become removed. With non-metallic inclusions thus eliminated, ingots with good internal cleanliness result.
The trouble with the conventional rotary ingot-making is in that it has been conducted on the top pouring principle (as disclosed in the Japanese Patent Publication Nos. 11113 of 1961 and 39597 of 1971). When operated in this way, the surface of the molten metal in the rotating mold becomes parabolic, under the effect of centrifugal force, rising toward the mold walls and dipping in the center. As a result, more violet splashing occurs than in the stationary top-poured casting, damaging the ingot quality to a greater extent. To reduce such vigorous splashing, a splash preventing cylinder is inserted in the mold, but the result is not quite satisfactory.